1. Field of the Invention
The present invention relates to the field of LED structures, and more particularly to the LED structure applied to a backlight source and capable of generating a white light with the CIE1931 chromaticity coordinates D65 standard while taking both NTSC effect and luminous efficacy and performance into account.
2. Description of the Related Art
In general, a semiconductor light emitting device has a physical property of emitting a light with a narrow spectral distribution, unlike the white light (or natural light) such as the sunlight is composed of lights with a broad spectral distribution. However, human eyes are situated in an environment having sunlight most of the time, so that the effect similar to the daylight illumination is desired in a design of an artificial light source for illumination and display. At present, LED components are used for designing and selecting a light source for illumination or backlight modules, but the LED components have the property of just emitting a fixed color light only, so that the principle of mixing colors is used for achieving the white light display or illumination. For example, a Japanese company, Nichia, proposed to coat a layer of YAG phosphor onto InGaN blue light chips, and the YAG phosphor is excited by the yellow light and the blue light complementarily to generate a white light. Regardless of using the independent RGB chip to mix the three primary colors to form the white light or using the principle of complementing two primary colors to produce different color lights from phosphors in order to form the white light, it is a major improvement to produce a white light effect similar to the general sunlight illumination effect. At present, the COMMISSION INTERNATIONALE DE L'ECLAIRAGE, CIE has defined standard specifications of D55, D65 and D75 for sunlight, so that display or illumination manufacturers may use these standards as a basis for the design of their products.
In the area of displays, a liquid crystal display (LCD) module does not emit light, and thus it is necessary to have a backlight source and a RGB color filter to achieve the color reproduction. Therefore, the spectral position of an original light source such as the white light of a backlight source is adjusted directly to improve the NTSC color effect of the display products, or a color filler of a different thickness or a different material is changed at a later manufacturing stage. For example, a conventional LED backlight source including a blue LED together with a yellow phosphor has the best luminous efficacy and incurs a low cost. Since white light is simply composed of blue light and yellow light, therefore the white light has less red light and green light which lead to a low NTSC effect. In addition, the technology blue light LED becomes mature, so that it is necessary to change yellow phosphor to green phosphor and red phosphor according to the color mixing theory of chromaticity coordinates in order to improve the NTSC effect and maintain using a single blue light LED as an active light source to excite the phosphor to produce green light or red light. Even though the NTSC effect can be improved by changing the composition of the white light, the brightness drops while the NTSC effect is improved due to the common red phosphor such as (Ca,Sr)AlSiN3:Eu2+ or Sr2Si5N8:Eu2+ has a specific range of absorption wavelength. The emission wavelength of a traditional yellow phosphor is usually limited within a range of 540 nm-570 nm, so that an emission wavelength of the red phosphor must be greater than 650 nm to achieve an NTSC effect of 80%. To improve the NTSC color range, it is necessary to increase the emission wavelength of the red phosphor theoretically. To increase the NTSC effect up to a level greater than 80%, it is necessary to change the yellow phosphor to a green phosphor (having an emission wavelength smaller than 540 nm). However, there are two sides to every coin. The aforementioned way of improving the NTSC effect will cause a drop of the overall brightness. To improve the brightness while selecting a bigger blue LED chip, the cost and power consumption are increased.
The following table shows that a nitride red phosphor with a conventional 1113 structure such as (Ca,Sr)AlSiN3:Eu2+ and a common green phosphor are used for an experiment, wherein the EU concentration of either Ca or Sr may adjust the excitation wavelength. The higher the concentration, the longer the wavelength. Since the purity of the green light is acceptable, therefore the overall NTSC color effect is not bad. However, the luminous efficacy in such condition can just achieve 70% of the brightness performance only, when compared with the standard lumen-to-watt ratio of 120 lm/W (or a luminous flux of 120 lumens per watt).
BrightnessCombinationFormulationStandard 120 lm/WNTSCG + R530 nm + 650 nm69.80%83.33%G + R530 nm + 660 nm68.72%84.13%G + R520 nm + 650 nm67.92%84.83%G + R520 nm + 660 nm66.82%85.33%G stands for green phosphor; and R stands for red phosphor.
If the green phosphor is substituted by the yellow phosphor, and the red phosphor is still the 1113 structured nitride, the brightness performance can be improved significantly, but the NTSC effect drops drastically as shown in the following table.
BrightnessCombinationFormulationStandard 120 lm/WNTSCY + R560 nm + 630 nm97.00%69.38%Y + R550 nm + 630 nm92.70%73.16%Y + R540 nm + 630 nm85.60%76.56%Y + R540 nm + 650 nm84.50%77.35%Y + R540 nm + 660 nm81.10%78.25%Y stands for yellow phosphor, andR stands for red phosphor.
Therefore, the present invention intends to improve both NTSC effect and brightness performance by using a method totally different from the conventional method and selecting a yellow phosphor with an emission wavelength greater than 540 nm and a red phosphor with an emission wavelength approximately equal to 630 nm to achieve the NTSC effect over 80% without the need of changing the original packaging condition to a larger blue LED while taking the overall LED brightness into consideration. The invention nut just lowers the overall manufacturing cost only, but also improves the color gamut and luminous efficacy or performance.